Vehicle manufacturers are increasingly employing electronic control units which operate in response to signals from inertia switch impact sensors. Electronic controls are already common both in door locking and fuel management systems. This move to electronic processing of sensor signals and the use of driver stages or relays to perform power switching has prompted the development of smaller and smaller low power switches and transducers which are frequently required to be small enough to be directly mountable within electronic control modules.
Known inertia switch impact sensors for this purpose comprise a ferromagnetic ball; a dished first contact having a circular portion of smaller diameter than the ball, for supporting the ball in a rest position, and an upwardly inclined outer portion extending from the circular portion; a second contact extending around a circle for engagement by the ball, on movement of the ball away from its rest position as a result of impact by the vehicle, to complete an electrical path between the two contacts; and a magnet disposed below the ball and spaced from the first contact.
One way to adjust the strength of the magnet so as to ensure that the sensor operates correctly is to fit the magnet in a fully magnetized condition and then to demagnetize the magnet until the restraint it exerts on the ball is reduced to the required level.
In mass production, when magnets are not demagnetized, to avoid this additional process step, in different production runs where magnets of the same strength are required to hold the balls with different retaining forces, trouble is encountered unless each magnet is positioned precisely in relation to the first contact. Also, as a result of the manner in which magnetic field strength varies with distance, it has been found necessary to space the magnet a relatively large distance from the first contact since, if the magnet is too close to the first contact, small deviations from the correct position of the magnet cause large variations in the force exerted by the magnet on the ball. This is important because the position of the magnet depends on dimensional imperfections in at least two parts: the member supporting the magnet and the magnet itself. Thus, in practice, it is normal to leave a relatively large air gap between the magnet and the first contact so that any imprecision in the positioning of the magnet will result in only minor variation in the force exerted by the magnet on the ball.